Weeds – threats to native ecosystems

We all know about the threats that predators like possums, rats and stoats pose to New Zealand's native ecosystems, but did you know different exotic weeds are threatening our ecosystems too?

The Parliamentary Commissioner for the Environment Simon Upton was concerned at the growing risk weeds pose to our environment. To determine the scale of the problem and what is being done about it, he decided to investigate.

The report examines how weeds in Aotearoa New Zealand harm our native ecosystems and reviews if the system for managing these weeds is fit for purpose.

There is a silent invasion of our forests by plants like wild ginger. Vines like banana passionfruit, old man’s beard and climbing asparagus are strangling forest understories. Russell lupins are carpeting braided rivers and wilding conifers covering hillsides. If left to their own devices, these weeds will transform our ecosystems beyond recognition.

Rt Hon Simon Upton, Parliamentary Commissioner for the Environment

What is a native ecosystem weed?

In the report, weeds are defined as exotic plants introduced to New Zealand and growing where they are not wanted. However, this can mean different things to different people. Whether a weed is a plant out of place depends on where it grows and the various social, economic and environmental benefits or risks it represents to us personally or as a group. One person’s treasure can be another person’s nuisance.

In te ao Māori, the idea of native plants as ‘good’ and exotic plants as ‘bad’ is not straightforward. What is more important is whether the plant is negatively impacting the mauri or life force of a place.

Protecting Papatūānuku is paramount, and bare ground is considered a scar on her body that needs to be clothed and protected. In this world view, any green covering is better than no green blanket at all. Such a blanket can help with erosion issues, provide quick-growing shelter and habitat for a range of native animals and improve freshwater quality or sometimes these plants can exacerbate fire risk or destroy habitat.

Weeds can also be viewed in te ao Māori as plants that disrupt the natural balance that Papatūānuku needs to be well. Therefore, the way a plant relates to other species becomes the key issue. If an exotic plant interferes with another species in a major way, this can weaken the mauri of the place, and the integrity of the whole ecosystem suffers.

Russell lupins, for example, are valued by many for their colourful flowers that produce picture postcard landscapes. To some farmers, their ability to fix nitrogen from the atmosphere and provide nourishment to the soil without the need to apply manufactured fertiliser makes them valuable pasture plants. However, lupins are also weeds that can spread rapidly. They form dense stands in the gravel beds of braided rivers and bush areas, creating problems for the native species that live there and altering ecosystem processes.

How big is the exotic weed problem?

Over 25,000 exotic plants have been introduced to New Zealand. Of these, nearly 3,000 species currently grow in the wild. For comparison, there are about 2,300 plants native to New Zealand, and only 31 introduced land mammals have been able to survive and thrive in the wild.

Problems can arise when exotic plants escape from where they’ve been planted and spread uncontrolled across the landscape with unintended impacts in new settings. The pool of potential garden escapees is vast, and at current rates, around 20 exotic plants establish in the wild every year.

These plants spread in various ways. Seed may be dispersed far and wide by birds or the wind, and our actions can also move plants around such as dumping of garden waste, tramping and moving materials or equipment.

How much damage can weeds do to native ecosystems?

Weeds can harm native ecosystems in many ways. The severity depends on the type of weed and the native ecosystem it is invading. In some cases, the impact is dramatic and obvious – such as when vines like banana passionfruit or old man’s beard smother native bush, killing native plants below and even sometimes causing the entire canopy to collapse. Aquatic weeds like hornwort can choke lakes and rivers.

Take climbing asparagus for example. Climbing asparagus (Asparagus scandens) can grow throughout the understorey of native forest, halting regeneration of native plants and scrambling up the trunks of trees and shrubs. When it has overwhelmed the canopy, it completely transforms the forest. It reproduces sexually through seeds and can also resprout from small fragments of plant, making it a very successful invader. You can see the damage it can do in Northland where it dominates forests.

Wild ginger (Hedychium sp.) is another one. It can infiltrate intact forests, forming dense rhizome beds that exclude native species usually found growing in the understorey. Over time, it prevents regeneration and causes the forest canopy to collapse.

Some impacts are less visible but still serious – for example, wilding conifers prevent native plants from growing by changing the soil chemistry and, together with gorse, make ecosystems more vulnerable to fire due to highly flammable foliage and wood.

The invasion of native ecosystems is helped by how easily many weeds can grow and thrive in the open habitats that dominate much of our landscapes. As well as outcompeting native plants, many exotic plants also thrive and spread in disturbed areas of bare earth – scars on the landscape caused by fire, floods, erosion or excavation.

Key report findings

Some of the key findings from the report:

• There are 25,000 exotic plant species already in New Zealand, and hundreds of these are actively causing harm to our native ecosystems.
• There is no comprehensive, up-to-date record of all the exotic plant species in New Zealand, including where they are, what harm they’re causing and how they are managed (if they’re managed).
• Exotic plants are mostly managed under New Zealand’s biosecurity system. However, weeds that impact on our native ecosystems tend to be a lower priority compared with other biosecurity threats like plant or animal diseases (think kauri dieback or Mycoplasma bovis) or animal pests (think possums, stoats and rats) or weeds that affect our productive land. The incentives to act on native ecosystem weeds are weaker or non-existent. Further, biosecurity measures are often focused at the New Zealand border. This means that the weeds that are already here in the country don’t get the attention required.
• Currently, not enough is being done to prevent exotic plants from escaping from gardens into the wild or to adequately address the risks from the plants that have already escaped (the plants that have ‘jumped the garden fence’).
• Surveillance of weeds is patchy, and reports from the frontline often depend as much on luck as systematic surveillance.
• Government organisations like the Department of Conservation can only manage weeds on government land like reserves and national parks. However, the status of a reserve or a national park does not automatically protect from weed invasion. Further, weeds don’t respect property boundaries and easily cross between public and private land. If not managed on private land too, weeds can continue to provide a constant source of seeds.

Recommendations

The report is clear that we will never get rid of all the weeds in New Zealand and we will need to keep weeding some plants for a long time to protect the native ecosystems we value.

These are some of the recommendations in the report to ensure native ecosystems and species are better protected from weeds:

• Clarify desired outcomes and help align national, regional and local efforts to allow for better guidance about which plants to manage, where and how they are to be managed and by whom.
• Any process to prioritise which weeds to tackle needs to be based on the risk each weed poses to native ecosystems. These risks include:
  • how likely a weed is to invade a given location
  • how well established or spread the weed is in a given location
  • the degree of harm it can cause to ecosystems once there
  • the potential for further spread
  • what will happen once a weed has been removed has to also be considered – removing weeds can create “weed-shaped holes” where an empty space can become fertile ground for other weeds to move in
  • Most of these risks are determined by the physical attributes of the plant – such as how the plant reproduces and how it can be dispersed.
• Improve existing weed information systems to provide everyone managing weeds with a single authoritative and publicly accessible database of all exotic plants in New Zealand.
• Improve monitoring and surveillance of emerging weeds.
• Establish an expert team to scan for emerging risks from new exotic plants that may be tomorrow’s weeds.

Weed action on the ground

The Parliamentary Commissioner for the Environment states, “Defeating weeds will not be achieved by top-down policies alone. Much of the weed control happening every day throughout the country is being done by passionate community groups, iwi, hapū and landowners.”

In the chapter ‘Down in the weeds…’, the report profiles several grass-roots initiatives of community and individual action against weeds. One of these (from page 201 of the report) looks at the work of rongoā practitioner Rob McGowan.

Taking action is a valuable addition for science learning. Action taking as part of an inquiry model can provide opportunities for students to develop valuable skills. Action taking provides students with a sense of hope, they are empowered to make a difference and this is linked to wellbeing. Learn about ways in which students and schools can take action for conservation in this recorded PLD webinar.